A wide variety of food products, such as pasta, beans, corn, peas, juices, sauces, and other vegetables and fruit, are processed by blanching or cooking prior to being frozen, packaged and shipped. Generally, there are two types of industrial blanchers: the steam belt blancher, as exemplified by U.S. Pat. No. 4,942,810, issued Jul. 24, 1990 to Zittel, et al., and the rotary blancher, examples of which are disclosed in U.S. Pat. No. 5,146,841, issued Sep. 15, 1992 to Zittel, and U.S. Pat. No. 5,632,195, issued May 27, 1997 to Zittel.
The steam belt blancher uses a conveyor belt to move food products through a chamber to subject the food product to a continuous, controlled temperature treatment in steam. Although the steam belt blancher has been heretofore commercially successful because it offers the advantage of using only steam, which is known to be a more efficient heat transfer medium than hot water, it suffers from having a great many moving parts, and can be expensive and relatively slow in operation. Moreover, it is not sealed at its inlet and outlet which means that the pressure within cannot be greater than the pressure outside.
Pressure cookers have also found use in commercial food processing applications because they are sealed and operate at a pressure inside greater than the pressure outside, i.e. ambient pressure. By operating at a pressure greater than ambient, food product inside a pressure cooker is heated more quickly and efficiently. Unfortunately, pressure cookers process food in batches and thereby cannot operate continuously because they must be periodically opened to remove processed food product and to reload new food product that is to be processed. When opened, nearly all of the heated, pressurized atmosphere within the pressure cooker escapes wasting a great deal of energy. Moreover, valuable cooking time is also lost because the pressure cooker cannot operate until it is loaded and because it takes time after loading for the atmosphere within the pressure cooker to once again reach the desired temperature and pressure. In commercial food processing, this start-stop cycling dramatically reduces the efficiency and desirability of pressure cookers. Hence, pressure cookers are only used in limited commercial applications, mainly because blanchers can operate nearly continuously and can process much larger loads of food product during operation than pressure cookers.
Prior art rotary blanchers typically use hot water, or a combination of hot water injected with steam as the heat transfer medium. However, because rotary blanchers churn and tumble the food product through a turbulent hot water bath, they are capable of processing a larger volume of food product at greater speed than a comparably sized steam belt blancher or pressure cooker.
In a rotary blancher, food product is introduced into an inlet end of an elongate cylindrically shaped drum, which is rotatably mounted in a generally cylindrical, open-top tank. The tank has a lid for enclosing the drum that can be opened for maintenance and cleaning. The drum has a helical auger running lengthwise through the drum that rotates during operation for transporting the food product from the drum inlet opening on one end to an outlet or discharge opening at the other end. The drum cylinder typically has sidewalls that are perforated to contain the food product in the drum while allowing a liquid heat transfer medium to come in contact with the food product as it is transported by the auger through the drum. As the food product moves from the inlet end of the tank to the outlet end, the time it remains in the tank, i.e., the residency time, is controlled to ensure that the food product is properly cooked or blanched.
While rotary blanchers have been improved in many ways, it has been believed to be heretofore impractical to blanch or cook only using steam because steam leakage would be so substantial that it would be too costly. Prior art rotary blanchers have been at best only partially steam-tight and have not been steam-tight about their entire periphery. It is known that at least one prior art commercial rotary blancher has a spaced apart pair of steam-tight water seals located where the sidewalls of the tank and cover meet with each seal extending longitudinally the length of the tank. Each water seal consists of a trough carried by the tank sidewall trough that receives one of the longitudinal edges of the lid sidewall. During operation, water partially fills the trough to create a seal between the trough and sidewall edge.
Such seals are effective at preventing the escape of steam along the longitudinal sides of the tank. However, significant amounts of steam can still escape along the seam where the endwalls of the lid and tank mate as there are no seals at these locations. In addition, steam can escape through the drum journal openings in the tank and lid endwalls, since clearance is provided around the drum journals to allow the drum to rotate freely. Steam can also escape from the drum inlet and outlet openings where food product is introduced and discharged since both are open to the atmosphere outside the blancher.
To prevent moisture from escaping from the atmosphere inside the blancher, some atmosphere within the blancher typically is continuously vented during operation to help create a negative pressure or vacuum within the blancher such that the pressure inside the blancher is less than the pressure outside the blancher. For example, for a blancher having a diameter of 5 feet and a length of 24 feet, about 1,500 cubic feet per hour of atmosphere would typically be evacuated from inside the blancher during operation. Despite this, some moisture and steam still leak from the blancher. Moreover, while this rate of venting is suitable to safely minimize increasing the humidity of the plant in which the blancher is operating using either water or a combination of water and steam, it is not suitable to prevent or suitably minimize steam leakage where such a rotary blancher is using only steam as the heat transfer medium.
While a small amount of steam leakage does not present a significant problem in the case of conventional hot water blancher operation, as steam is used to a greater extent to achieve higher temperatures and more rapid heat transfer, steam leakage can become a significant problem where only steam is used as the heat transfer medium. One of the major expenses involved in processing food product in a blancher is the cost of energy needed for generating the steam or heating the water used to blanch the food product. Thus, loss of steam results in energy loss that wastes money. In addition, if steam is allowed to escape into the factory it can condense and drip onto food product which can result in its contamination and loss. Moreover, increased humidity in the factory caused by the escape of significant amounts of steam can contribute to worker fatigue, and will result in generally less than desirable working conditions, slippery and hazardous factory floors, and accelerated corrosion of food processing equipment within the factory. Minimizing the amount of steam that escapes from a blancher is thus highly desirable.
Rotary blanchers using only steam as a heat transfer medium are believed to be capable of processing food product as much as 10-20% faster than prior art rotary blanchers. Rotary blanchers using only steam at a pressure greater than the ambient atmospheric pressure are believed to be capable of processing food product as much as 600% faster than prior art rotary blanchers. However, due to the problems noted above, it has not heretofore been commercially achievable.
What is needed, therefore, is a rotary blancher that is sufficiently steam-tightly sealed so as to enable the rotary blancher to blanch or cook by heating using only 1) a vapor, 2) a heated gas, or 3) a combination of vapor/gas and a liquid with the vapor/gas inside the blancher at a pressure greater than the ambient pressure outside the blancher. What is also needed is a rotary blancher that permits food product to be substantially continuously introduced into the blancher and substantially continuously discharged from the blancher without disturbing the seal of the blancher enabling the blancher to maintain a pressure inside the blancher that is greater than the ambient pressure outside the blancher.